This invention relates to systems for storing data, and in particular to volume replication in storage area networks. Large storage systems are now well known in which primary and secondary storage systems are provided for a data center or other similar need. The primary and secondary volumes are sometimes also referred to as production and replica volumes, respectively. In such storage systems the primary and secondary storage systems (or subsystems) are linked, for example, using a communications link such as an optical fiber, the Internet, or other well known data communication pathway. The purpose of linking such systems is to assure that an extra copy of data is available at a remote location. This assures that a natural disaster, a failure of the data link, a failure of one or the other storage systems, or other event, does not preclude the system from having useful data.
The volume replication function has become popular as a fundamental function to prevent the need to stop a computer application when a storage system failure occurs. Such failures would otherwise require a delay while the data is restored from back-up tapes or the like. It is also important to have a restore function from the secondary volume to the primary volume when any data loss occurs on the primary volume. It is also a desirable use of this function to reuse the replica volume for other purposes, for example, reporting, testing, back-ups, migration to new systems, etc., without impacting the primary volume.
When the link between the primary (production) volume and the secondary (replica) volume is suspended due to a planned or unplanned event, each of the primary and secondary volumes will be used for their own purposes, i.e. the data on each will not necessarily correspond to the data on the other volume. In other words, by loss of the link between them, the system is at least temporarily deprived of any logical method to synchronize the two volumes. Furthermore, once suspended, subsequently occurring writes will occur between the primary and secondary volumes as each volume is updated for other purposes. Once the integrity of the system is reestablished, i.e., the data link is restored, then it is desirable to be able to resynchronize the primary and secondary volumes.
Examples of prior art approaches to restoring the volumes include a description in the IBM Redbook, see, e.g. pages 23 to 27. Another solution is found in the Hitachi Freedom Storage Software Solution Guide at pages 28 to 38. In addition, the copy function in Hitachi systems is also described in U.S. Pat. No. 6,408,370 and in European EP 0981091 A2.